It is not for instance a function of muscles to appreciate touch, heat or cold, and they are never required to do so ; it has also been shown in the living human being that the hollow internal organs such as the intestines, are not sensitive to inappropriate stimuli and that they can be cut, pricked or burned in the con scious subject without any sensation being registered. But tissues other than the skin have their appropriate functions and must, for their own welfare, be appreciative of excessive or inadequate activity or stimulation. Thus the muscles must be able to gauge the strength of opposing forces and must have a limit to the strength with which they can contract and to the number of times they can contract within a certain period. Extreme stretching or extreme contraction of a muscle, as in cramp, causes pain.
The pain in hollow muscular organs, like the stomach, the bowel, the bladder, the bile-ducts and the arteries, also depends upon excessive tonic or contractile action. Thus when a gall stone is impacted in the bile duct—a very narrow tube—a great increase in tension in the wall of the duct necessarily follows and severe pain results until the stone is passed or relief brought by other means. The mucous membrane (or inner coat) and the serous (or outer) coat of all the hollow organs is, as has been indicated, insensitive to the ordinary stimuli which the skin ap preciates, but a bubble of wind and still more some pathological stricture by evoking muscular contraction and increased tension of the muscle-fibres in an attempt to dislodge the bubble or to overcome the obstruction, may cause pain which is referred to the deep situation of the part affected. Solid organs like the brain and liver are apparently insensitive to ordinary stimuli painful in the case of the skin. A bullet, a blade or a needle may traverse them and no hurt be felt in their substance. The delicate covering membranes of these organs, however, are responsive to inflamma tion or stretching, and still more to a combination of these, giving the intense headache of meningitis and the pain and tenderness of an engorged or inflamed liver.
The lung is not sensitive but its delicate pleural investment cer tainly is, as sufferers from a pleurisy know only too well. Arteries are sensitive but—as with other hollow organs—stretching or spasm are the effective causes of pain. Bone is not sensitive but its transparent covering, the periosteum, is certainly so, especially when its tension is increased by the swelling attendant upon bruising or inflammation. Direct stimulation of a sensory nerve causes pain and other unpleasant sensations along the course of the nerve. The rolled or injured ulnar nerve (or "funny bone") experiences pain at the point of stimulation and simultaneously an electric twitch of pain radiates down the inner side of the arm to the little finger. In sciatica, an inflammatory nerve-pain, the unpleasant sensations are often felt throughout the course of the nerve to its ultimate distribution.
In all these types of pain the sensation is located in conscious ness at the part affected, but there are other types of pain which are spoken of as "referred" or "sympathetic." These depend upon the fact that the nerve-supplies to certain deep-seated structures or organs and certain superficial structures have a common origin. In some cases of tooth-ache pain is experienced in the ear or some area of the scalp which may even become tender to touch as though the skin were inflamed, and yet it shows no signs of local disease. This pain and hypersensitiveness is explained by the circumstance that the nerves to the diseased tooth and the sympathetically affected skin have a common receiving station in the ganglion of the fifth or trigeminal nerve. In angina pec toris the pain is often referred down the left arm, because the same segment of the spinal cord supplies both this region and the heart with its immediate arterial offshoots. In gall-bladder in flammation the pain is often referred to the shoulder-blade, in inflammation of the diaphragm to the shoulder itself, by reason of a similar sharing of segmental nerve-supplies. The tenderness which may develop in these surface areas of sympathetic pain may be of very great value to the physician in his in flammations. as i to the nature and localization of deeply-seated and invisible n flammations. The stimuli in the deep-seated structures potent for the production of referred or sympathetic pain and hyper sensitiveness are either some violent mechanical insult as in the case of an impacted gall-stone or kidney-stone, or some more sustained irritation such as is caused by active inflammation in the wall of the organ.
Numerous observations may be made by the physician with regard to a particular pain and each of these may have a special value in helping him to draw conclusions as to its seat of origin, its nature and its ultimate causes. Thus he can elicit information in regard to :—(I) Its character (whether gnawing, aching or burning) ; its severity (whether easily borne or agonising); (3) its exact situation; (4) its localization or extent of diffusion; (5) its paths of sympathetic propagation or reference; (6) its duration; (7) its frequency; (8) the special times at which it is liable to occur ; and, finally, (9) and ( o) its aggravating and relieving factors. From such an inquiry he is able to endow pain with a diagnostic significance which, in the civilised community, by providing indications for its medical or surgical relief, further serves to enhance its natural value as a protective agent.
See also SKIN, SENSORY FUNCTIONS OF THE.